Writing a basic journey

To get our feet wet we’re going to start with a simple user journey that does a few things - first it will ask the user for their date of birth, secondly it will calculate the number of days the user has been alive and finally it will return their date of birth as a string.


You will need to import the core uniform library, and you will need cats. You do not need to import any interpreters when merely defining the journey.

import ltbs.uniform._
import cats.implicits._

We will also be using higher-kinded types, here we have enabled this as an import -

import scala.language.higherKinds

We’re going to use the old java.time.LocalDate in our program too along with some code for formatting. This is specific to our example here and not likely needed for your project.

import java.time._, format._

Type declarations

Next we have the type declarations for the ‘tells’ -

type TellTypes = Long :: NilTypes

In this instance we want to be able to present the user with Long values. This means that any interpreter running our journey must know how to present an Long to the user (for example a web-based interpreter would probably need to know how to render this datatype into HTML). We’ll use this to tell the user the number of days they have been alive for.

If our journey needed to be able to present Addresses, Names and lists of Colours to the user we would assign TellTypes to Address :: Name :: List[Colour] :: NilTypes.

Sometimes we don’t need to present anything back to the user in which case we would just use NilTypes on its own.

Next we need a similar type declaration for the ‘asks’ -

type AskTypes = LocalDate :: NilTypes

Here we are saying that an interpreter must know how to prompt the user to enter a java.time.LocalDate. This will be used to prompt the user for their date of birth.

Journey definition

Now we can create our main journey body.

def dateOfBirth[F[_] : cats.Monad](
  interpreter: Language[F, TellTypes, AskTypes]
): F[String] = {
  import interpreter._

  for {
    dateOfBirth <- ask[LocalDate]("date-of-birth")
    daysAlive = LocalDate.now.toEpochDay - dateOfBirth.toEpochDay
    _           <- tell[Long]("days-alive", daysAlive)
  } yield dateOfBirth.format(

Notice that this is very similar to a tagless final program - except that we are taking our TellTypes and our AskTypes as additional parameters into our interpreter (telling the interpreter what we expect it to support), and that the interpreter.ask method accepts a type parameter which controls its output type.

We created TellTypes and AskTypes to supply them into our journey. We could have just inlined these types but generally it is useful to create type aliases as it can help with debugging and makes your code a bit clearer.

Notice that we are creating a String type in our journey, but we don’t need to have this type declared in either TellTypes or AskTypes.

The journey itself defines the user-journey or interaction you wish to model. It is completely abstract at this point and the interpreter gets to decide how to represent the questions.